In such system the driving element can be a water or wind driven turbine, an internal combustion engine or other drive engine, and the driven element can be an electric generator (dynamo) or other apparatus which should be kept accurately at a predetermined rotational speed without substantial oscillations.
Although the invention can be applied generally to any system of the kind referred to it is particularly well suited for application to wind power stations. In such power stations the generator preferably is a synchronous generator which should be driven at the synchronous rotational speed in order to generate electric power at the predetermined frequency, usually 50 or 60 cps. As far as a synchronous generator is concerned the rotational speed thereof is maintained at the value corresponding to the frequency of the network to which the generator is connected, but due to rapid and abrupt changes in the wind velocity and direction and in the generator load it cannot be avoided that oscillations occur in the rotary system, due to the fact that the input torque supplied by the turbine deviates from the value corresponding to the torque required by the generator at the actual speed and load. Such oscillations create transient currents in the generator circuit with accompanying energy losses.